Open cell flexible polyurethane foams are widely used in a variety of applications. They offer high resiliency and load-bearing properties, rendering them useful for applications such as furniture cushions and carpet underlay. Open-cell flexible polyurethane foams have traditionally been prepared from TDI (toluene diisocyanate) polyisocyanate components. However, for many high load-bearing applications, such as carpet underlay, TDI-based foams typically have a density of &lt;3 lbs/ft.sup.3, and are subject to fatigue where load-bearing capacity is reduced. This is particularly acute for carpet underlay, where high traffic areas can suffer from premature carpet underlay fatigue.
MDI has been used to make high-density open cell flexible polyurethane foams. These foams can provide dense foams (&gt;3 lbs/ft.sup.3), which can offer advantages in fatigue resistance compared to TDI-based foams. TDI-based foams cannot be readily prepared with densities above 3 lbs/ft.sup.3. MDI-based foams also offer good tensile strength (&gt;35 psi) and tear strength (&gt;2.75 pi). However, the increased reactivity of MDI-based polyisocyanate components can cause various problems. Depending on the nature of the polyol used in combination with the MDI-based polyisocyanate, the foam produced may exhibit high hardness rendering it inappropriate for cushioning use. Also, the necessary degree of open cell structure in the foam may not be achieved.
One significant problem encountered in the preparation of MDI-based open cell flexible polyurethane foams has been the formation of so-called "creme balls" in the foam preparation equipment. Creme balls are regions of polyurethane foam that form and remain in the foam preparation equipment, causing fouling and reducing the yield of good quality foam from the equipment. This problem is particularly severe in trough-type slabstock foam dispensing machines. It is toward the problem of providing open-cell flexible polyurethane foams exhibiting good fatigue properties and reduced tendency for fouling of foam dispensing equipment that the present invention is directed.